1. Field of the Invention
The present invention relates to processes for purifying, by the wet method, fumes containing nitrogen oxides.
2. History of the Related Art
In such processes, a washing liquid is brought into contact with the fumes to be purified in a washer. The dust in suspension in the gases is transferred into the washing liquid and, at the same time, the gaseous pollutants are absorbed by the washing liquid depending on their solubility and the characteristics of the washing liquid. The flowrate of the latter necessary for purifiying the fumes generally being high, the washing liquid is recycled in the washer with a portion thereof being drained to extract from the washer/recycling circuit trapped pollutants.
In the case of nitrogen oxides emitted by combustion processes, about 95% of the nitrogen oxides are in the form of nitrogen monoxide NO whose solubility is very low; and a simple method of washing, with a washing liquid containing lime or limestone as is conventionally carried out for sulfur oxides, does not enable the nitrogen oxides to be absorbed in a significant quantity, the proportion of nitrogen dioxide NO.sub.2 rarely exceeding 5% of the total of the nitrogen oxides present. This is why different processes have been proposed for oxidizing the NO into NO.sub.2 to thus allow the trapping thereof. All such processes employ an oxidant whose consumption may be much greater than that strictly necessary stoichiometrically due to the presence for example of dissolved sulfur dioxide which tends to be oxidized into sulfur trioxide. Another drawback of such processes is that nitrates are produced which pose a virtually insoluble problem concerning the treatment of the concentrated fluid being drained.
It is an object of the present invention to overcome these drawbacks by judiciously using the properties of the other pollutants trapped in the washer. Experience has surprizingly shown that the oxidation of NO and NO.sub.2 may be combined with a reduction of NO.sub.2 into N.sub.2 due to the reducing properties of dissolved SO.sub.2.